The present invention relates to a safety switch for electric rolling doors which automatically cut off power supply from the electric rolling door, when two metal sheets thereof are forced into contact with each other, as the electric rolling door is stopped by an object during its down stroke.
Regular electric rolling doors are commonly equipped with a safety switch, which automatically cuts off power supply when the electric rolling door is stopped by an object during its down stroke. FIG. 1 shows a safety switch for this purpose, which is comprised of two overlapped metal sheets and an insulative sheet retained between the metal sheets. When the metal sheets and the insulative sheet are arranged together, they are covered with rubber by a molding machine. The insulative sheet has a plurality of through holes so that the metal sheets can contact each other when the safety switch is vertically compressed. This safety switch structure is still not satisfactory in function. The drawbacks of this safety switch structure are numerous and outlined hereinafter. Because the effective induction area of the safety switch is limited to a narrow area right below the metal sheets corresponding to the holes on the insulative sheet, the safety switch does not work if it is stopped by an object at the corner area during the down stroke of the electric rolling door. Furthermore, when the metal sheets and the insulative sheet are arranged together and covered with rubber by a molding machine, the molded device is rolled up, and then cut to the desired length subject to the size of the electric rolling door. Because the safety switch is rolled up before it is cut to the desired length, the metal sheets tend to be wrinkled or deformed. When the metal sheets are wrinkled or deformed, the sensitivity of the safety switch is affected. In order to increase the sensitivity of the safety switch, an elastic casing may be used (see FIG. 3). This elastic casing is molded from elastic material having inside ribs, and a receiving chamber above the inside ribs for mounting the safety switch. This arrangement greatly increases the installation cost of the safety switch. Furthermore, the safety switch cannot be effectively induced to cut off power supply from the electric rolling door when the electric rolling door is stopped by an object in the opposite vertical sliding grooves along which the electric rolling door slides.